Conventional flow cytometric (FCM) analyses and FCM lifetime analysis are being used to elucidate mammalian sperm chromatin structural changes in response to physiological and radiological stimuli, particularly those stimuli that lead to apoptosis. Most of the abnormal spermatogonia following irradiation appear to be undergoing apoptosis but the morphologic features are not typical, suggesting apoptosis may have different appearances in different cell types. These studies are designed to develop new methods for evaluating different modes of cell death in testis.